Tuesday, July 19, 2011

La Niña Update-Forecast, July 2011

We are currently (July 2011) emerging from a strong La Nina which impacted the weather patterns over the U.S. during the past Winter and spring.

From The International Research Institute for Climate and Society:

The following graph shows forecasts made by dynamical and statistical models for SST in the Nino 3.4 region for nine overlapping 3-month periods. Note that the expected skills of the models, based on historical performance, are not equal to one another. The skills also generally decrease as the lead time increases. Thirdly, forecasts made at some times of the year generally have higher skill than forecasts made at other times of the year--namely, they are better when made between June and December than when they are made between February and May. Differences among the forecasts of the models reflect both differences in model design, and actual uncertainty in the forecast of the possible future SST scenario.

Most of the set of dynamical and statistical model predictions issued during late May and early June 2011 predict neutral ENSO conditions from the June-August 2011 season currently in progress, through the end of the calendar year. However, a minority of models call for a re-emergence of La Nina conditions going forward, while a few other models suggest the development of El Nino conditions. For the June-August season the probabilities for La Nina, neutral and El Nino conditions are estimated at 9%, 84% and 7%, respectively. At the time of preparing this, the SST observations in the NINO3.4 region had returned to the climatological average (the middle of the ENSO-neutral range), with an area-averaged weekly anomaly of 0.0 C in the most recent week. Current predictions and observations indicate probabilities of 14% and 15% for La Nina conditions during the August-October and September-November periods, respectively, with corresponding probabilities for El Nino conditions of 15% and 15%. Probabilities for ENSO neutral conditions are substantially greater than those for La Nina or El Nino during all seasons, hovering near 70% for all seasons going forward into early 2012.